The Holsteen
Welcome to the Holsteen, a magical place that was described in 1862 and still captures the imagination today. The Holsteen is the collective name for the eight large sandstones you see here.
The stones with peculiar cavities resemble a prehistoric structure, such as dolmens or menhirs. But the way the stones are placed together is pure coincidence. The holes are also entirely natural: they are the remains of animal tunnels or of roots in the sand before it petrified.
In 1922, Professor Joseph Hamal-Nandrin explored this site and made a remarkable discovery. Take a closer look at the northernmost stone to find out what was so special.
A unique geological phenomenon
The name Holsteen, which first appeared in scientific literature in 1862, is a place name for the terrain on which a number of large pale sandstone blocks are located that outcrop on the western edge of the Kempens Plateau in the east of the municipality of Zonhoven. In the past, a wide variety of explanations were given for the presence of these blocks. We now know for certain that these blocks were not brought from elsewhere by nature or by man but were formed and remain here completely naturally. The blocks were formed by soil processes in quartz-rich sea sand, which was deposited here in the early Miocene period, approximately 20 million years ago. This is easy to recognise by the layers that are still clearly visible in the stones.
Sandstone
On this sandstone block you can still clearly see how the sea sand was deposited in layers (lines in the stone). Because the lines are not horizontal, we know that this stone was once tilted. However, we do not know how this happened. ©Gemeente Zonhoven
The geological processes that led to their formation have only recently been discovered. The hardening of sand into sandstone occurred through silicification, a diagenetic process of cementing of sand grains by silica during warm, dry periods after the sea had retreated. In the case of the Holsteen, this cementing only took place after the sand had first been bleached by humic acids. These acids originated from an overlying brown coal layer that formed here many millions of years ago, during a warm, humid period, in a coastal swamp covered with swamp cypress trees and which has now completely disappeared due to erosion.
No human cause needs to be sought for the presence of numerous round hollows and perforations on the sandstone blocks. These are mainly fossil root tunnels, which have been further eroded over time by water and wind. These root tunnels are also proof of the now disappeared marsh vegetation.
The Holsteen and its surroundings were protected as a landscape by Royal Decree of 11.09.1967. Only eight sandstone blocks are visible. But there may be other, usually smaller specimens buried under the sand.